Coherent population trapping is one of the techniques used in atomic clocks in order to acquire an atomic frequency as a stable time base. Such atomic frequency acquisition devices or atomic clocks comprise a miniaturized gas cell containing a metallic vapor such as, for example, Cs or Rb. Normally, this gas cell is kept at a temperature of about 100° C. so as to realize a certain gas pressure. A laser emitting at a wavelength which fits an optical transition in the corresponding atom, for example, 852 nm for Cs, is arranged to direct the laser beam into the gas cell filled with the corresponding atomic gas. At the same time, the laser emission is modulated by a stable electronic oscillator running at a frequency which fits half the hyperfine transition of the corresponding atom, e.g. 4.6 GHz for half the Cs splitting. This frequency is used as the time base for the atomic clock. A detection photodiode measures the attenuation of the laser beam after passage through the cell. Electronic circuitry adjusts the electronic oscillator to the frequency of the atomic hyperfine transition based on the photodiode signal which shows a minimum at the target frequency. This minimum is due to coherent population trapping which refers to the fact that a fraction of the atoms become trapped in a coherent superposition of the two ground states forming the hyperfine transition, which does not absorb light due to destructive interference between the transition probability amplitudes of the optical transitions of these ground states to the optical excited state.
D. K. Serkland et al., “VCSELs for Atomic Clocks”, http://www.sandia.gov, describe the use of VCSELs (VCSEL: vertical cavity surface emitting laser) for atomic clocks relying on such coherent population trapping. The physical background of this technique is also explained in this document. Serkland et al. use a gas cell filled with Cs which is excited by a VCSEL emitting at a wavelength of 852 nm. The laser output is frequency-modulated at 4.6 GHz, thereby obtaining frequency modulation side bands at +/−4.6 GHz from the carrier optical frequency of 352 THz (852 nm). The power of the laser beam after passage through the gas cell is measured by a photodiode which is arranged on the opposite side of the gas cell. When the modulated VCSEL is tuned to the approximately 1 GHz wide absorption resonance and the modulation frequency is tuned in the vicinity of 4.6 GHz, a narrow increase of the transmitted optical power is observed at exactly half of the 9.2 GHz hyperfine frequency. The local oscillator for generating the 4.6 GHz modulation is locked to this narrow resonance, yielding a frequency which is exactly half of the Cs ground state hyperfine splitting.